Processes and devices used for electro photographic printers wherein a laser scan line is projected onto a photoconductive surface are known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on a pre-charged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which switches on and off as it moves, or scans, across a photoreceptor.
Commonly, the surface of the photoreceptor is selectively imaged and discharged by the laser in locations to be printed. On-and-off control of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling of the laser source. A common technique for effecting this scanning of the beam across the photoreceptor is to employ a rotating polygon mirror surface; the laser beam from the ROS is reflected by the facets of the polygon, creating a scanning motion of the beam, which forms a scan line across the photoreceptor. A large number of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet, as in the well-known process of xerography.
While several exposure systems have been developed for use in electro photographic marking, one commonly used system is the raster output scanner (ROS). A raster output scanner is comprised of a laser beam such that the laser beam contains image information, a rotating polygon mirror having one or more reflective surfaces, a motor polygon assembly, etc. Some raster output scanners employ more than one laser beam. Usually in motor polygon assembly (MPA), errors may occur during manufacturing. Based upon these errors erratic beam reflectivity may occur from each facet in a ROS Imager MPA assembly that is then passed on to ROS outputs as dysfunctions in critical applications.
Laser scanning is based on a technique achieving both start-of-scan detection and dynamic beam intensity regulation in a multiple laser beam raster output scanner using a photodetector. The raster output scanner includes a source, or sources, of a plurality of laser beams or arrays, a rotating polygon having at least one reflecting facet for sweeping the laser beams to form a scan line path, and a photodetector for receiving illumination from the multiple laser beams and for converting those beams into beam-dependent electrical currents. The raster output scanner further includes a scan detection circuit for producing a start-of-scan signal, and a beam intensity circuit for producing an electrical output signal which depends upon the beam intensity of each laser beam. Optionally the raster output scanner also can include an optical fiber 102 that collects a portion of the light flux in the sweeping laser beams which directs the light flux onto the photo detector. Referring to FIG. 1 (prior-art) the top view 100 of a raster output scanner used in the electro photographic printing machine is illustrated. The raster output scanning assembly 100 can include a plurality of laser diodes or array(s) 150 and 151 which produce laser beams 103 and 104, respectively, are modulated according to image data from the data source and laser driver 152. The image data from the data source and laser driver 152 might originate from an input scanner, a computer, a facsimile machine, a memory device, or any of a number of other image data sources.
The purpose of the data source and laser driver 152 is to excite lasers 150 and 151 with modulated drive currents such that the desired electrostatic latent image is interlaced on the photoreceptor in precise registration with uniform exposure. The output flux from laser diodes 150 and 151 are collimated by optical elements 154, reflected by fold mirror 156, and focused on reflective facets 157 of rotating polygon 158 by cylindrical lens 160. The facets of rotating polygon 158 deflect the beams which are then focused into well defined spots focused on the surface of photoreceptor 10 by scan lens elements 162 and 164. As the polygon rotates, the focused spots trace parallel raster scan lines on the surface of the photoreceptor. The sensor network 106 is positioned in the scan path to collect light flux from beams 103 and 104 at the beginning of the scan. Optionally, the input end of the optical fiber 102 is positioned in the scan path to collect light flux from beams 103 and 104 at the beginning of the scan. The optical fiber 102 transmits the intercepted flux to the sensor network 106. Beam intensity signal 110 and the start of scan signals are configured from the sensor network 106 to the data source and laser driver 152. The synchronized input 122 is configured to the sensor network 106.
The present inventor has recognized a drawback of prior art of laser scanning is with lack in effectively controlling the output intensity variation of exposing beam(s) of a rotating polygon type image forming apparatus using control marks formed on a rotating surface portion of a polygon member or a motor polygon assembly. Ideally, control marks can be read by a reader during rotation of the polygon member, and the information read from the control marks is used to control the modulation of the exposing beam of the image forming apparatus to expose evenly spaced, uniformly sized, precisely oriented, geometrically straight scan lines of pixels on a photosensitive member. The control marks can include pixel clock information, intensity correction information, error correction information about individual facets of the polygon member, and motor speed control information.